This invention relates to improvements in materials for laminating printed circuits at low pressures on flat bed presses.
In the manufacture of flexible and flat cable circuits, layers of high dielectric strength plastic films with low shrinkage and good stability at high temperatures are first combined with copper or other conductive metal by an additive or subtractive process and electrical circuits are produced by etching or deposition of these metals on the plastic surface. These circuits are then covered by a layer of similar adhesive coated plastic dielectric film to protect and insulate the finished circuit Holes may be punched or drilled in the layers to allow for connections to the finished circuit.
The circuit assembly is then laid up with a protective cap in single or multiple layers (up to 20 plus) and then laminated under heat and pressure, taking extreme care to register the circuits and drilled holes. This process is normally performed in a flat bed laminating press by the operator of the press, using pad materials consisting of multiple layers piled on between the steel caul plates and the finished circuit Typically, the first layer, next to the steel plate, is a non-sticking release material such as a tetrafluoroethylene coated fiberglass cloth. The second layer includes multiple plies of kraft paper or equivalent cellulosic wood product; the third layer consists of one or more plies of a thermoplastic film; and the final layer consists of one or more layers of release materials such as tetrafluoroethylene or polyvinyl fluoride The resultant lamelliform pad may include 10 or more loosely piled layers of dissimilar materials, each having two sides presenting up to 20 opportunities for possible contamination. Because each layer is customarily handcut and laid up by hand, there is a high margin for error in the sequencing of dissimilar materials, resulting in defective circuits.
The pads are arranged in a prescribed order on either side of the circuit assembly with the release layer facing the assembly. Multiple circuit assemblies with associated pads are placed in the press to form a book, and the entire book is pressed in one operation to laminate the individual circuit assemblies. When the laminating process is complete the pads are stripped away and the laminated circuit assemblies are removed for further handling.
Among the problems found in the prior art process were erratic results and lamination failures due to errors in selecting and laying up the multiple plies of dissimilar materials, hot spots appearing in the laminate creases, pockets of gas and other contamination trapped in the pad, lateral shifting of the pad materials and the circuit laminate, and moisture absorption by the cellulosic pad.
U.S. Pat. No. 4,690,845, issued Sept. 1, 1987 to Thomas M. Kloss, Jr., discloses an invention which alleviates the foregoing problems by providing a unified stratiform press pad sheet which is used in place of the loosely assembled multi-layer pads of the prior art. The Kloss press pad sheet is used primarily in applications in which the circuit assembly is laminated at temperatures in the range from 180 to 400 degrees F.
More recently research efforts have been directed toward applications in which the circuit assembly is laminated with new dielectric materials which provide improved rigid and flexible circuit boards but which require higher lamination temperatures. This in turn has created a need for a unified press pad which is functional at the higher lamination temperatures. The new requirement involves temperatures as high as 560 to 600 degrees F.
Accordingly, it is an object of the present invention to provide a unified stratiform press pad sheet which offers the advantages provided by the Kloss press pad but which is functional both at the conventional lamination temperatures and at the higher temperatures required to accommodate the newer dielectic laminating materials.